Coating metal articles



Dec. 29, 1959. l.. GTREAT 2,919,208

` COATING METAL ARTICLES Filed Nov. s. 1955 United States Patent COATINGMETAL ARTICLESV Lyle G. Treat, Ferguson, Mo., assignor to The DowChemical Company, Midland, Mich., a corporation of Delaware ApplicationNovember 3, 1955, Serial No. 544,801

3 Claims. (Cl. 117-113) The invention relates to methods of providing apigmented coating, such as a paint coating, on metal objects which arerelatively long compared to their width and thickness and have a uniformcross section from end to end, e.g. extrudes.

In the metal industries, the problem frequently arises of providing auniform coating of paint or the like on long articles especially thoseproducedby the extrusion method. As is well known, the extrusion processpermits making metal objects of almost any length in a wide variety ofcross-sectional shapes which are uniform from end to end. Oftentimes theshape of the cross section of an extrude involves great complexity ofform making it difficult, if not impossible, to coat uniformly withpaint, or paint-like materials such as an enamel, by any conventionallyavailable method including dipping.

While dipping is the preferred conventionally available method ofapplying a paint coating to long objects as extrudes, it is noteffective to produce a coating of `uniform thickness economically andquickly. Accordingly, it is the principal object of the invention toprovide an improved method of applying avpaint or like pigmented coatingmaterial to metal objects having a length which is relatively greatcompared to their girth. A particular object is to provide an improvedmethod of the character indicated which can be executed rapidly,produces a coating of uniform thickness, and is economical of coatingmaterial. Other objects and advantages will appear as the description ofthe invention proceeds, reference being had to the accompanying drawingwhich illustrates schematically the sequence of steps of the method.

The invention is predicated upon the discovery that l by adjusting theviscosity of the liquid pigmented coating material within certaincritical limits, hereinafter set forth, long objects of uniform crosssection can be coated very rapidly and uniformly by an end-wise dip inone direction in the coating material followed by a rapid withdrawal ofthe vertically disposed object, at least partial drying, and thenanother endwise dip in the opposite direction followed by a rapidwithdrawal of the vertically disposed object from the coating material,the rate of withdrawal in each direction being critical and related tothe viscosity of the coating material as hereinafter disclosed. Thus bya proper control of the viscosity of the pigmented coating material, andby a rapid rate of endwise withdrawal of the object (that is withdrawalwith the longitudinal axis of the object vertical) from the coatingmaterial, the object becomes coated with a relatively thin lm of thecoating material, the thickness of the coating tapering slightly fromthe lower end of the object where the film is the thickest to the upperend where the film is the thinnest. After allowing a sufficient time forthe lm produced by the first endwise dip to set,

the so-coated object is turned end for end and given a second dip in thecoating material and again similarly withdrawn rapidly in the verticalposition. The second endwise dip puts another film, somewhat thickerthan the iirst, of the coating material on the object, the lilm .1 cupat 80 F.

ICC

as before tapering in thickness from the end last leaving the coatingmaterial (bottom end in second withdrawal), where the second film is thethickest, to the opposite end where the second lm is the thinnest. Sincethe two films have substantially the same taper but in oppositedirections, the resulting double film makes a single coating of uniformlthickness from one end of the object to the other. Since the rate atwhich the coating material can be applied is limited only by the rate atwhich the article can be introduced endwise into a bath of the coatingmaterial, the rate of withdrawal, and the setting time of the coatingmaterial between dips, uniform coat-` ings can be produced with extremerapidity.

In applying the method, various coating materials may be used which arerecognized in the trade as paints, enamels, primers, and the like,comprising a drying liquid vehicle and a pigment in suspension in thevehicle. The term pigmented liquid coating material is used herein andin the appended claims to mean any coating material of the foregoingcharacter. In practicing the invention, conventional liquid pigmentedcoating materials may be used but require'appropriate thinning so that aparticular viscosity'is achieved within the operating temperature of themethod which is between about 55 and 110 F.

In defining the'viscosity whichthe liquid pigmented coating materialshould have, for use in accordance with theinvention, reference is hadto the Standard Method of Test for Viscosity of Paints, Varnishes, andLacquers by Ford Viscosity Cup as set forth under ASTM designation:D1200-54 adopted, 1954, using a No. 4 Ford In accordance with theforegoing test, the preferred viscosity of the coating material is about14 seconds. However, good results may be had with vscosities as high as16 and as low as 12 seconds.

The bath of coating material may be adjusted as to viscosity usually bythe addition of a suitable thinner when the conventional pigmentedcoating materials are too viscous for use in the invention or by bodyingwith a suitable bodying agent if the coating material is too thin.Suitable thinners are organic liquids which are of low viscosity andcompatible with the coating material. EX- amples of thinners aretoluene, methyl isobutyl ketone, methanol, ethanol, acetone.

As aforesaid, the rate of vertical endwise withdrawal of the object fromthe pigmented coating material is criticaland may range from a minimumof 10 feet per minutey to as much as 60 feet per minute withtemperatures of 55 to 110 F. A preferred operating temperature for thecoating material is 70 to 80 F. using a withdrawal rate of 30 to 40 feetper minute and a viscosity of No. 4 Ford viscosity cup of 14 seconds.

The following example is illustrative of the practice of the inventionas applied to coating metal extrudes with a primer.

Example A commercial primer, comprising 35 percent solids, a vinylbutyral resin, and zinc chromate pigment, was diluted with from about175 parts of thinner per 1-00 parts by volume of primer. The thinnercomprised toluene, methyl isobutyl ketone, methanol, and acetone andreduced the viscosity, as measured by a No. 4 Ford cup, in accordancewith the aforementioned viscosity test, to about 14 seconds at 80 F. Theso-thinned primer was then ready for use. An extrude about 9 feet longand having a uniform cross section of complex profile from end to endwas hung from one end with gripping jaws attached to a cable of a hoist.The extrude was lowered vertically by the hoist into a verticallydisposed cylindrical tank, containing the aforesaid diluted primer atF., to a depth sufcient to completely immerse the extrude in the primerexcept for a small portion of the extreme upper end where the grippingjaws were secured to the extrude. The rate of immersion (not critical)of the extrude in the primer was about 50 feet per minute. The immersedextrude was then withdrawn at the rate of 33 feet per minute (in thecritical range).` The so-withdrawn extrude thereby had acquired a thincoating of the primer. The so-coated extrude was set aside in air for 40minutes to dry. The so-treated extrude was then turned end for end andthe gripping jaws were removed and attached to the opposite end. Theextrude was then again lowered vertically into the primer as before butin the opposite direction at the rate of about 50 feet per minute (notcritical) and then immediately withdrawn from the primer at the rate of33 feet per minute (in the critical range). The resulting coated extrudewas then set aside in air for 20 minutes to dry after which it was ovenbaked at 275 to 300 F. for 20 minutes. After baking the coated extrudewas found to have a primer coating of substantially uniform thickness of0.5 mil from end to end along the portion between the extreme ends Wherethe gripping jaws were secured.

The foregoing example is illustrative of the uniformity and speed of thecoating method in spite of the double dip or immersion procedure. Speedis attained because the rate of immersion is not critical although fast,being limited only by ones ability to get the work into the coating bathand obtain contact between the work and the coating material andby thetime required for Withdrawal which, in accordance with the invention, israpid being critically correlated with the viscosity of the coatingbath, and by the time required between the first withdrawal and secondimmersion to set the lirst coat stifliciently to prevent it from beingdisturbed during the second immersion.

The actual over-all time of coating an article, in accordance with theinvention, except for the time allowed for drying which, of course,varies with the nature of the pigmented coating material used, isexceedingly short as already indicated mainly because the withdrawalrate is from to 60 times faster than the rate of withdrawal inconventional dip coating operations in which the withdrawal rate is lessthan 1 foot per minute. The method, because of its speed, has thefurther advantage of requiring fewer tanks or vessels for holding thecoating material, and the required floor space for the operation isgreatly decreased for the same output of coated articles.

Although the invention has been described more par'- ticularly inconnection with providing a uniform pigmented coating on metal extrudes,it is to be understood that the method is not limited to coatingextrudes. It is manifest that any other metal articles which arestraight, relatively long, and of uniform cross section, such as tubes,rods, bars, strips, bands, structural shapes, including angles, Tsections, H sections, Z sections, moldings, and the like, may be coatedby the method.

I claim:

1. The method of coating a straight metal object having a greater lengththan girth and of uniform cross section from end to end with a pigmentedliquid coating material which comprises lowering the vertically disposedobject endwise into a bath of pigmented liquid coating material so as tototally immerse the same while in the vertically disposed position,withdrawing the so vertically disposed object endwise from the coatingmaterial at a rate between 10 and 60 feet per minute, drying the coatedobject while so vertically disposed so as to set thereon the lm ofcoating material, inverting the so ilrned object end for end loweringthe vertically disposed so inverted object endwise into the coatingmaterial so as to again totally immerse the same while in the verticallydisposed position, again withdrawing the so vertically disposed objectendwise from the coating material at a rate between 10 and 60 feet perminute, and then drying the resulting coated object while heldvertically disposed, the pigmented liquid coating composition having aviscosity at`80" F. of from 12 to 16 seconds as measured by the Fordviscosity cup No. 4, and a temperature of to 110 F.

2. The method according to claim 1 in which the pigmented liquid coatingmaterial is a paint.

3. The method according to claim l in which the pigmented liquid coatingmaterial contains zinc chromate pigment.

RefrncesCited in the le of this patent UNITED STATES PATENTS Y 15,5111HDVi June 10, 1856 1,462,608 Marshall July 24, 1923 1,774,453 RaicheAug. 26, 1930 1,804,991 Johnson May 12, 1931 1,952,935 Miller Mar. 27,1934 2,127,548 Boyle ---a Aug. 23, 1938 2,181,305 Myers Nov. 28, 19392,260,282 Grint Oct. 28, 1941 2,683,286 Lenhart July 13, 1954 OTHERREFERENCES Protective Coatings for Metals, R. M. Burns, 1955, publishedby Reinhold Publishing Corp., 430 Park Ave., New York, New York, pages414 and 415.

1. THE METHOD OF COATING A STRAIGHT METAL OBJECT HAVING A GREATER LENGTHTHAN GIRTH AND OF UNIFORM CROSS SECTION FROM END TO END WITH A PIGMENTEDLIQUID COATING MATERIAL WHICH COMPRISES LOWERING THE VERTICALLY DISPOSEDOBJECT ENDWISE INTO A BATH OF PIGMENTED LIQUID COATING MATERIAL SO AS TOTOTALLY IMMERSE THE SAME WHILE IN THE VERTICALLY DISPOSED POSITION,WITHDRAWING THE SO VERTICALLY DISPOSED OBJECT ENDWISE FROM THE COATINGMATERIAL AT A RATE BETWEEN 10 AND 60 FEET PER MINUTE, DRYING THE COATEDOBJECT WHILE SO VERTICALLY DIOSPOSED SO AS TO SET THEREON THE FILM OFCOATING MATERIAL, INVERTING THE SO FILMED OBJECT END FOR END LOWERINGTHE VERTICALLY DISPOSED SO INVERTED OBJECT ENDWISE INTO THE COATINGMATERIAL SO AS TO AGAIN TOTALLY IMMERSE THE SAME WHILE KIN THEVERTICALLY DISPOSED POSITION, AGAIN WITHDRAWING THE SO VERTICALLYDISPOSED OBJECT ENDWISE FROM THE COATING MATERIAL AT A RATE BETWEEN 10AND 60 FEET PER MINUTE, AND THEN DRYING THE RESULTING COATED OBJECTWHILE HELD VERTICALLY DISPOSED, THE PIGMENTED LIQUID COATING COMPOSITIONHAV-